But its still a glitch with minecraft, the server, even if it is modded, if you put a mob on the players head and get rid of the mods, and go back to vanilla is still in effect, and im sure if it was possible to use commands to put a mob on youre head (which i believe it is) it would still show in vanilla,
Confirmed
It isnt a server issue, it has to do with the tab list in minecrafts code, they didnt make the tab lists for players long enough to fit the max characters of a minecraft username,